1. Field of the Invention
The present invention generally relates to a color solid-state imager and, more particularly, is directed to a camera that digitally processes the image signal.
2. Description of the Background
A color solid-state imager is known in which a four-color filter made-up of the three complementary colors, cyan (Cy), magenta (M), yellow (Ye), plus green (G) arranged in a checkered pattern is combined with a solid state imager element. An example of such solid-state imager is found in Japanese Patent Laid-Open Gazette No. 59-161989.
FIG. 1 shows a color filter 10 in which cyan and yellow filter elements 1c and 1y are alternately arrayed on an n.sup.th row, and magenta and green filter elements 1m and 1g are alternately arrayed on the (n+1) row. The rows then alternate between lines of cyan and yellow filter elements and rows of magenta and green filter elements. The pattern arrangement of the cyan and yellow filter elements in the n.sup.th, (n+2), and (n+4) rows are the same, however, the pattern arrangements of the magenta and green filter elements in the (n+1), (n+3), and (n+5) rows are opposite in phase, that is, they are offset by one filter element every row.
In FIG. 2, a color solid-state imager system is comprised of an analog signal system, shown generally at 20, and a digital signal processing system, shown generally at 30. In the analog signal system 20, a solid-state imager element 21 is provided in an opposing relation to the color filter 10 that is formed as shown in FIG. 1. This solid-state imager element 21 includes light receiving picture elements arranged in a two-dimensional manner, that is, in an X-Y matrix form, and transfer elements for transferring signal charges of respective picture elements, for example, when imager element is a charge-coupled device (CCD). An output signal from solid-state imager element 21 is supplied through a correlative double sampling circuit 22 to an automatic gain control amplifier (AGC) 23. An output from the AGC amplifier 23 is sampled and held by a sample and hold circuit 24, and an output from sample and hold circuit 24 is supplied to an analog-to-digital (A/D) converter 25, in which it is converted into a digital signal of, for example, 10 bits.
In the digital signal processing system 30, the digital signal from A/D converter 25 is supplied to a low-pass filter 31 and to a bandpass filter 32. An output from low-pass filter 31 is supplied to a luminance signal processing circuit 33, in which it undergoes necessary signal processing, such as, aperture correction, gamma correction, and the like. On the other hand, an output of bandpass filter 32 is supplied to a matrix circuit 34, in which it is processed to provide three primary color signals, red, green, and blue (R, G, and B). The red (R), green (G), and blue (B) primary color signals are supplied to a color signal processing circuit 35, in which they undergo necessary signal processing, such as white balance adjustment, gamma correction, encoding, hue correction, and the like. In the conventional color solid-state imager, since the signal processing is performed in a digital fashion as described above, the color solid-state imager can be made compact in size and requires relatively low power.
The output from luminance signal processing circuit 33 and the output from color signal processing circuit 35 are supplied to digital-to-analog (D/A) converters 36 and 37, respectively, wherein they are converted into an analog luminance signal Y and an analog color signal C, respectively.
In the operation of the color solid-state imager described above, the light reflected from an object is spatially modulated by each of the filter elements of color filter 10 and each modulated light beam is photoelectrically converted by each respective light receiving element (not shown) of solid state imager element 21 and then sampled. In the sampling operation, a sampling frequency fsp is selected to be four times as high as the color subcarrier frequency fsc, for example, fsp may equal 14.3 MHz. Further, a repetitive frequency of the color filter 10 is selected to be about 1/2 of the sampling frequency fsp.
In the odd-numbered field, two rows of the light receiving picture elements corresponding to the n.sup.th row and the (n+1) row; the (n+2) row and the (n+3) row; and so on, of the color filter 10 are horizontally scanned at the same time to sequentially generate the photoelectric-converted outputs at every two columns. That is, the output signals are calculated at every four picture elements of two rows and two columns. In the even-numbered field, two rows of the light receiving elements corresponding to rows (n+1) and (n+2); the (n+3) and N+4) rows; and so on of the color filter 10 are horizontally scanned at the same time to sequentially generate the photoelectrically converted outputs at every two columns.
The light receiving picture elements corresponding to the cyan and magenta filter elements 1c and 1m of n.sup.th row and (n+1) row derive signal components (B+G) and (R+B), respectively, and the light receiving picture elements corresponding to the yellow and green filter elements 1y and 1g derive signal components (R+G) and G, respectively, as shown in FIGS. 3A and 3B.
Further, the light receiving elements corresponding to the cyan and green filter elements 1c and 1g of the (n+2) row and the (n+3) row, respectively, of color filter 10 derive signal components (B+G) and G and the light receiving picture elements corresponding to the yellow and magenta filter elements 1y and 1m color filter 10 derive signal components (R+G) and (R+B), respectively, as shown in FIGS. 3C and 3D.
In the horizontal scanning of the odd-numbered and even-numbered fields, a total sum Ss of the output signals generated at every 4 picture elements of 2 rows and 2 columns is expressed as: Ss=(2R+3G+2B). Thereafter, the signal is processed by low-pass filter 31 and the resultant signal is a luminance signal that is uniform and compensates any lens distortion.
If a sum of the output signals of each of the columns is calculated at every four picture elements of two rows and two columns on the n.sup.th row and the (n+1) row in the odd-numbered field and a difference between the sum signals of each of the columns is calculated, then a differential signal Sdb is expressed as: EQU Sdb=[(B+G)+(R+B)]31 [(R+G)+G]=2B-G
Similarly, if a sum of output signals of each of the columns is calculated at every four picture elements of two rows and two columns on the (n+2) row and the (n+3) row and a difference between the sum signals of each of the columns is calculated, then a differential signal Sdr is expressed as: EQU Sdr=[(B+G)+G]-[(R+G)+(R+B) ]=-(2R-G)
Further, the same differential signals Sdb and Sdr are obtained on the (n+1) row, the (n+2) row, the (n+3) row and (n+4) row of the even-numbered field.
The differential signals, Sbd and Sdr, are subcarrier chrominance signal components that are formed on the basis of the spatial modulation performed by the respective filter elements of color filter 10. The repetitive frequencies thereof become one half of the sampling frequency fsp, that is, twice as high as the color subcarrier frequency fsc, for example, fsp may equal 7.16 MHz.
Nevertheless, in the previously proposed color solid-state imager the level of the analog carrier chrominance signal component delivered from the AGC amplifier 23 is decreased, for example, to 1/4 to 1/2 of the level of the luminance signal component, so that when the analog signal is digitally converted by the A/D converter, even according to the linear quantization process, the accuracy of the carrier chrominance signal is about 8 to 9 bits for the standard input level equivalent, which is, for example, 10 bits.
Further, when a dynamic range that is three times as wide as the standard input level is maintained by the gamma correction circuit, the high-level region is compressed, so that with the standard input level the accuracy of the carrier chrominance signal component is decreased to about 6 to 7 bits.
As a result, fine gradation of the color signal cannot be obtained and a so-called false contour occurs in the reproduced picture, in which there are step changes in the brightness of certain colors in place of the desired gradual change in brightness. Furthermore, a satisfactory signal-to-noise (S/N) ratio cannot be obtained due to the quantization noise produced by the analog-to-digital conversion.